『Abstract
　Distribution and mobility of phosphorus in soil and sediment are usually studied by sequential extraction. In the extraction procedure, a sample is treated with a series of reagents to distinguish the phases to which phosphorus is associated such as carbonate and iron or manganese oxides, etc. There have been a number of extraction schemes presented for phosphorus. At present, all of the existing schemes are carried out batchwise. Phosphorus contents derived from all sequences are operationally defined and depend on experimental conditions.
　An extraction procedure, which is a continuous-flow-based technique, was recently proposed by our group for metals in soils and sediments. The extraction is carried out in a closed chamber through which extractants are passed sequentially. In this paper, the system was investigated using the extraction scheme of Hieltjes and Lijklema to study distribution of phosphorus in three certified reference materials (CRMs). A number of fractions were collected for each reagent for subsequent colorimetric determination. The results are compared with those obtained from a batch extraction. The summation of phosphorus contents of all phases were compared with the certified values and with the values obtained from total digestion. These results have demonstrated that the continuous extraction system developed is also applicable for fractionation of phosphorus. Advantage and disadvantage are discussed.

Keywords: Phosphorus; Sequential extraction; Continuous-flow system』

1. Introduction
2. Experimental
　2.1. Chemicals
　2.2. Sample
　2.3. Sequential extraction
　　2.3.1. The batch procedure
　　2.3.2. The continuous-flow procedure
　2.4. Procedure for phosphorus dissolution of sample and residue
3. Results and discussion
　3.1. Validation of the method for phosphorus dissolution
　3.2. Extractogram given by the continuous-flow extraction
　3.3. Fractionation of phosphorus by the continuous-flow and the batch procedures
　3.4. Comparison between the continuous-flow and the batch procedures
　3.5. Clogging of the in-line filter in the extraction chamber
4. Conclusions
Acknowledgements
References